Try these in the order listed.

1. Scrub with soap and water

2. Use a paper towel moistened with rubbing alcohol (isopropyl)

3. Use a paper towel moistened with MEK (methyl ethyl ketone)

4. Use a cloth moistened with acetone or laquer thinner.
Warning: this chemical will dissolve the chip material and remove hotstamps and should be used gently and as a last resort.